1. Field of the Invention
The present invention relates generally to apparatus for dry placer mining, and, more particularly, to such apparatus for concentrating recoverable metallic products from gravel particulate wherein the separatory effects are the consequence of fluidization of the gravel and electrostatic retention of the metallic constituent on a moving belt.
2. Background Art
Dry placer mining, particularly for mining gold and silver from gravel deposits, has been known for many years. In an earlier application, which issued May 29, 1984 as U.S. Pat. No. 4,451,357 (hereby incorporated in its entirety by reference), applicant disclosed a machine which has proven highly successful for such mining operations. This is a compact, durable, and highly efficient device which concentrates metallic products from gravel particulate by employing a fluidizing air stream and an endless belt which is electrostatically charged by the air stream.
Inasmuch as the overall configuration of this earlier machine is somewhat similar to that of the improved machine of the present invention (and the improvements of the present invention are in some respects relative to the former), a general overview of these systems will be provided here. Accordingly, turning to FIG. 1, this shows a dry placer mining machine, designated generally as 10, which comprises generally a frame 12 which supports and guides an endless separation belt 14 about a closed, generally trapezoidal path surrounding a fluidization means in the form of an air box or plenum chamber, this being designated generally as 16. The separation belt 14 is caused to traverse the closed path by means of a drive system, designated generally as 18. As the belt moves about its path, ore or a particulate gravel mix containing metallic constituents is charged to the upper face of machine 10 along an upwardly inclined segment of the trapezoidal path, designated 20 in FIG. 1, while fluidization gas (most conveniently, air drawn from the surrounding atmosphere) is passed upwardly from the open mouth of plenum chamber 16 through the separation belt 14. As the belt moves upwardly along the inclined path, as represented by the arrow in FIG. 1, the gravel and sand components of the charge are fluidized by the air passing through the belt. During this fluidization, as a consequence of the structure of the belt 14 and its relationship to the plenum chamber 16, an electrostatic charge is developed proximate the separation belt 14. As a result of the combination of the fluidization of the gravel and the electrostatic charge which is developed on or proximate the belt, the metallic constituents are retained on separation belt 14 while the gravel is fluidized and flows downwardly across the inclined slope of the belt. The retained product is thereby enriched or concentrated in the relative proportion of metallic constituents vis-a-vis the original gravel. The concentrate, which includes heavy or dark sands, may then receive further refining treatment.
Since machine 10, at least in its smaller embodiments, is intended to be portable, the frame 12 is comprised of a base plate 22 which provides a solid foundation, and an upper frame structure designated generally as 24, this being pivotally mounted on the base plate 22. The plate 22 includes a pair of stanchion members 26, and these support the upper frame 24 about pivot pins 28 so that the frame 24 and associated portions of machine 10 may pivot from a transportation configuration, where the face 20 is in a generally horizontal orientation, to an inclined operational configuration such as that shown in FIG. 1. Any convenient means for pivoting the frame on the pivot pins may be employed; FIG. 1 shows a pair of hydraulic cylinders 30 which are conveniently employed to serve this purpose. Whatever means are used to apply the pivoting force, it is desirable that the same be capable of providing an adjustable angle of inclination to the upper face 20 of the belt, since this angle is an important factor in establishing the residence time of the gravel on the belt during the separation process.
The construction of belt 14 is of manifest importance to the efficient separation of the metallic constituents of the gravel charge. In the machine which was disclosed by the above-referenced earlier patent, the belt comprised three components, namely a woven metal mesh belt, a plurality of round riffle members, and a fabric member. The woven mesh belt was a steel mesh web which made up a series of upstanding loop elements, through which the round riffle members were inserted so as to extend transversely with respect to the upwardly inclined path of the belt, generally as shown in FIG. 1, so that these defined a series of transverse collection zones. The riffle members were relatively small diameter (i.e., 5/8 inch) steel rods, positioned within the mesh-to yield collection zones about 31/2 inches wide. The fabric member (which was positioned beneath the riffle members) comprised an intermediate layer of an air pervious polyester foam, sandwiched between lower and upper fabric layers. The upper fabric layer was made of a relatively fine woven cotton/polyester blend broadcloth, while the lower layer was made from a synthetic fiber fabric or batting having a fairly open weave, such as a coarse woven polyester batting of fibers such as those sold under the name Orlon.TM..
As noted at the outset, this previous machine has for the most part proven highly successful in the field. Nevertheless, its belt assembly (constructed in the manner described above) has exhibited a number of inefficiencies. Perhaps foremost amongst these has been the tendency for the electrostatic charge to dissipate from the belt assembly, thus reducing the ability of the belt to retain sufficient charge to efficiently separate the constituents of the gravel charge, and so this has led to the need to provide an auxiliary electrostatic charge generator. It has been discovered that this tendency to dissipate the electrostatic charge is linked to the use of various uninsulated metallic components in the construction of the belt, the steel rods forming the riffle members and the steel mesh web are excellent conductors which tend to quickly dissipate the charge, and the "flights" which form the upstanding borders of the belt assembly have also been constructed out of steel. Furthermore, it was found that the cotton/polyester blend broadcloth material which was used to form the uppermost layer of the fabric member of the belt, while performing admirably under very dry conditions, also tended to cause dissipation of the charge when conditions became damp, this apparently occurring due to absorption of water by the cotton component of the material.
Another inefficiency which was observed with the belt having the construction described above relates to the relatively small diameter of the riffle members which have previously been employed. In operation, it was noted that as the metallic constituents were collected on the belt, these tended to accumulate in "pockets" which were formed at the lower edge of each collection zone where the underlying fabric member met the riffle; in practice it was found that these pockets were insufficient in size and so tended to quickly overfill and lose their ability to retain the metallic particles.
Other difficulties which were encountered with the belt having the construction described above stemmed from the arrangement of the upstanding plates or "flights" which formed the borders of the belt. By way of illustration, the corresponding flights in the machine of the present invention are designated generally by reference numeral 32 in. FIG. 1. As was noted above, in the earlier machine these flights consisted of a series of thin metal plates, which aggravated the dissipation of the electrostatic charge from the belt. Another problem which was encountered with the metal flights was that these presented a serious safety hazard, inasmuch as gaps open and close between the adjacent flights where these pass over the sprockets at the ends of the inclined run of the belt, and an operator's hand or other body part might accidentally be received in these gaps and be severely cut.
It was also found to be difficult to form an effective seal to prevent the escape of air from the plenum chamber when using the belt having the prior construction. In an attempt to overcome this problem, an additional elongate sealing member (typically round or half-round in cross-section) was installed along the edge of the plenum, and then this was permanently compressed against the edge of the belt so as to form a seal; not only has this approach proven marginally effective in terms of forming the desired airtight seal, but the additional drag and friction which was generated due to the need to keep the sealing member partially compressed hindered the smooth operation of the machine, and in severe cases could cause tearing of the belt.
Other inefficiencies which were encountered with the previously-existing type of machine included the tendency of excessive undesirable mineral dust and other non-metallic particulate matter to accumulate on the belt, whether due to the electrostatic charge or simple adhesion, so that this was collected with the metallic constituents and so diluted the concentration of the recovered material. Also, in many applications it was found necessary to install a supplemental electrostatic charge generator in the plenum chamber inorderto augment the buildup of the electrostatic charge which occurs as the air passes through the belt material, reducing the overall economy of the operation; as was noted above, the need for this auxiliary generator was increased by the tendency of the belt to dissipate its electrostatic charge.
Accordingly, there exists a need for a dry placer mining machine of the type described above, wherein the construction of the moving belt of the machine reduces or eliminates dissipation of the electrostatic charge which is imparted thereto.
Furthermore, there exists a need for a machine having such a belt in which increased collection areas are formed where the riffle members and the fabric member of the belt meet at the lower edges of the collection zones.
Furthermore, there exists a need for a machine having such a belt in which the flights at the borders of the belt have a construction which reduces or eliminates the possibility of personnel being cut thereby.
Furthermore, there exists a need for a machine having such a belt in which the belt forms an effective seal to prevent escape of air along the edges of the plenum chamber without generating excessive friction and drag between the belt and chamber.
Furthermore, there exists a need for such a machine having means for dislodging and removing excess accumulations of undesirable mineral dust and other particulate matter from the surface of the belt prior to the concentrated metallic constituents being collected therefrom.
Still further, there exists a need for such a machine which is constructed so as to eliminate the necessity of using an auxiliary electrostatic charge generator to provide sufficient electrostatic charge on the moving belt assembly.